The invention relates to a device for the continuous filtration of fluids by means of the pressure differential between the inlet and outlet, as well as the removal of residual amounts of the slurry by means of a pneumatic pressure medium before opening the device, said device having
a plurality of filter plates having a slot on both sides for seating a planar filter medium and equipped with drainage channels for the filtrate;
a plurality of membrane plates having connections, bores and recesses for the admission of the pressure medium, which membrane plates are covered on both sides with an elastic membrane such that a slurry chamber or a filter cake chamber is formed between the filter medium and the membrane;
corresponding recesses in the filter plates and membrane plates which when installed form at least one admission channel for the slurry and one drainage channel for the filtrate, and also having
end plates, retaining fixtures and clamping fixtures to connect the filter plates, membrane plates and frames to form a filter pack that can later be disassembled.
The invention also relates to a filtration and cleaning method. From the aspect of its structural design, such a device belongs to the group of devices known as filter presses. However, it is used for filtration like a plate filter, in which the filtrate is forced through the filter medium by means of the pressure differential of the fluid between the inlet and the outlet.
A prior art example of a plate filter is disclosed in DE 39 06 816 C3. There are a plurality of filter plates arranged in parallel consolidated into a filter pack. When assembled, filter beds is pressed together between mating surfaces of the filter plates, which provide a seal between the slurry chambers and the filtrate chambers, and seal the entire filter pack to the outside.
A prior art example of a filter press is disclosed in DE 39 32 422 A1. The membrane plates alternately disposed between the usual filter plates comprise a circular frame with connections for the admission and drainage of a pressure medium, and a central wall, with a membrane attached to each side of a frame symmetrical to the central wall via a clamping ring. The axial length of the filter press is due to the cumulative width of the filter plates and the frame elements of the membrane plates. The space between the central wall of the membrane plate and the filter media attached to both sides of the filter plates is divided by the membranes into roughly equal sized suspension and pressure medium chambers. The frame elements have bead-like elevations extending along the axis of the filter device, over which elevations the membranes are stretched to improve their durability/stability when subjected to cyclic loads of up to 120 bar in these particularly critical areas. Because the membrane is stretched and arranged in this manner, the membrane is not extended but compressed when impinged by the pressure medium during the press process. As a result, the membrane never makes complete contact with the membrane plate when unpressurized, nor can the membrane make complete contact and lie flat against the filter plate when pressurized. Given the purpose of the generic device, the prior art filter press has an unnecessarily long axial length due to the pressure media chambers between the membranes and the central walls of the membrane plates. In other words, the space between the filter plate and the membrane plate that can be used as a slurry chamber is quasi halved because of the central position of the membranes, which under otherwise identical conditions also results in an oversized filter press.
The prior art also discloses filter devices in which pressurized membranes work in conjunction with filter cloths that must completely or partially match the change in shape of the membranes during filtration. In these cases, the membranes are equipped on the side facing the filter cloth with spacing cams so that the filtrate passing through the filter cloth can drain between the filter cloth and the membrane. Such membranes are unsuited for the intended application of the device according to the current invention.
The object of the current invention is therefore to refine a prior art device as described above for the filtration of fluids such that the residual losses can be significantly reduced compared to conventional plate filters without having to accept the disadvantages of prior art filter presses.
To achieve this object, the current invention teaches that the filter medium comprises sections of a sheet filter material mounted in the recesses of the filter plates; that the membranes, which when pressurized expand elastically to completely contact the filter plates or sheet filter material, have a smooth surface on both sides and make complete contact with the membrane plates when depressurized.
Sections of a proven and inexpensive sheet filter material can be used as the filter medium because the normal filtration process is the same as with conventional plate filters. Sheet filter material has a labyrinth-like depth filtration structure, which when used in conjunction with different surface charges permits both mechanical and adsorptive separation of particles along the relatively long path through the filtration medium. These materials, also known as filter beds, are special paperboards for the filtration of fluid media with the goal of removing coarse to super-fine particles, colloids, microorganisms and other undesirable components, and thus maintain the desired quality of the filtrate or to extract solid residues. These are manufactured using special paper machines, primarily Fourdrinier paper machines.
The raw materials are usually selected, bleached celluloses from conifer or deciduous woods of high purity, i.e. with high alpha content, selected diatomaceous earths and perlites, as well as manmade fibers such as polyolefin fibers, activated carbon, polyvinylpyrrolidone (PVP) or similar materials for special applications.
Most filter beds can be backed on the downstream side to prevent the loss of fibers during filtration without adversely affecting flowthrough. Certain approved resins can be used in small amounts to achieve specific effects (interfacial potentials, zeta potential).
The selection of raw materials, the processing thereof and the blending ratios together with other manufacturing parameters determine effectiveness and application.
The alternately disposed membrane plates are only activated when forcing residual amounts of the product from the slurry chamber prior to opening the device, and the remaining filtrate is forced by means of the pressurized membranes through the filter cake and the filter medium.
Because the slurry chamber is completely lined by the membrane when pressurized, it is possible to reduce the residual losses to nearly zero because the slurry chamber can be almost completely emptied by means of the membranes, and because the filter cakes are also mechanically pressed. The filter cake is solidified and nearly completely free of residual filtrate. The filter cake is almost completely dry and can be easily removed together with the filter medium after opening the filter device. The filter cake is thus prepared for further processing as is required in some filtration applications, such as the separation of blood plasma, for example.
A membrane that is smooth on both sides prevents filter cake constituents from sticking to the membrane, and also ensures that the drainage channels in the filter plates can be optimally covered when impinged with a fast-flowing detergent for cleaning.
The drainage channels on the filter plate are preferably vertical and horizontal grooves in the filter plate in the area of the groove for the sections of the sheet filter material.
The drainage channels on the filter plate are preferably vertical and horizontal grooves formed in the area of the slot in the filter plates for the sections of sheet filter material.
The horizontal and/or vertical grooves are preferably connected to at least one filtrate manifold in the interior of the filter plate.
At least one frame is located between each membrane plate and the filter bed to define and adjust as necessary the size of the slurry chamber. This makes it possible to adapt the size of the slurry chamber to the filtration job, in particular when the thickness of the filter cake is not the same for all filtration processes.
The frames can be polygonal, preferably square, and enclose both horizontally and vertically a slurry or filter cake chamber having a trapezoidal cross-section, whereby the boundary surface formed by the membrane plate is larger than the boundary surface formed by the opposing filter plate.
It is advantageous for the frame to have beveled inner surfaces to facilitate the expansion and contacting of the membranes when inflated. The bevel is preferably such that the frame thickness continuously decreases from the membrane side to the filter bed side. This prevents dead spaces because the inflated membrane can completely line the slurry chamber.
The membrane plate is preferably equipped with a support body on which the membrane is arranged. A compressed air duct is preferably located in the support body, which duct is connected via outlet openings to the two surfaces of the support body adjacent to the membranes. The compressed air duct is preferably branched. The recesses for the inlet and drainage channels in the filter plates are preferably in the area of the slots for the sections of sheet filter material.
The sections of sheet filter material can preferably be pressed together edge-to-edge between the filter plates and the frames to form a seal. The sections of sheet filter material are used to form a seal between the chambers containing the slurry and the filtrate, as well as to seal the device to the outside. The advantage of this is that additional seal elements are not required.
The measure also helps ensure that the membranes extend over the entire surface of the membrane plates and when installed are clamped together edge-to-edge between the membrane plates and the frames to form a seal.
The membranes preferably protrude over the uniform contour of the filter plates at least in the area of the connections for the pneumatic pressure medium. This configuration offers the advantage that the fasteners for the membranes are accessible from outside the device.
The compressed air connections of the membrane plates are located on the end face of the membrane plates, preferably outside of the vertical, longitudinal midplane of the device. This significantly aids side access to the compressed air connections by operating personnel during installation.
The filter plates can have a groove running around the outside on both sides at some distance from the edge for seating a seal, which seal can be considered a supplemental measure given the seal function already provided.
The filter plates, membrane plates and/or the frames are made primarily of plastic, preferably polypropylene or polypropylene copolymers so that FDA listing can be attained and the device can be used for the filtration of sensitive fluids. The filter device is therefore particularly suited for blood filtration.
The membrane is preferably made of a thermoplastic elastomer. The advantage of this material that it is very highly flexible, enabling the membrane to completely contact the contour of the slurry chamber.
The method for filtering fluids using the device teaches that actual filtration is by means of the pressure differential of the fluid between the inlet and the outlet, and that the membrane plate is impinged by a pressure medium, pressing the membranes against the filter cakes, filter media and filter plates to press out residual amounts of slurry and to dry and solidify the filter cake.
The method for cleaning the device teaches that the filter plates, membrane plates and possibly the frames without sections of sheet filter material are first assembled, the membranes are inflated until contacting or nearly contacting the filter plates, and finally a fast-flowing cleaning fluid is sent through the filtrate drainage channels in the filter plates.
It is possible to only inflate the membrane far enough to create a small space between the membrane and the filter plate with a correspondingly small cross-section so that high velocity cleaning fluid flows can be achieved. The flow velocity of 2 m/s required for optimal cleaning can be easily achieved.